This invention relates to a data transmission network, and more particularly to a data transmission network which is adapted for data transmission among many groups of equipment for which real time control is required.
In transportation equipment, such as railway vehicles, ships or airplanes, nuclear power plants, chemical plants, steel plant or a building system associated with elevators and devices for safety, there is provided various equipment which is collectively controlled.
In order to electrically connect such equipment, multi-wire cables or coaxial cables have been used. However, such a system becomes more complicated year by year as the system is provided with more improved functions, resulting in requirement of more increased electrical connections. It is difficult to satisfy the requirement by increasing the number of cables connecting the equipment, because of the cost, weight and space for mounting the cables and also because of restriction in reconstruction of the electrical connections.
For example, in a train composed of plural vehicles, most of the control signals are sent from the driver's compartment to various equipment mounted on the vehicles. Therefore, many through-wires are provided in parallel from the driver's compartment and pass through the vehicles and control signals are distributed to desired equipment through the wires.
It is also usual, in such a train system, to divide the whole system into a plurality of units, each unit including two to four vehicles and the control functions are provided in each unit. In this case, it is required to provide, in addition to the above-mentioned through-wires, a plurality of unit wires which interconnect the equipment of each unit.
Thus, the number of wirings needed for the vehicles was considerably increased and especially with recent progress of vehicle control, it has much increased and it is not unusual nowadays that the wirings provided through the vehicles comprise more than 300 lines. Furthermore, in order to perform intelligent driving of a train which includes monitoring the condition of each equipment, it is expected that more than several hundreds of signal exchanges are necessary. The conventional system has a problem in excessively increased weight, space, and mounting cost for wirings in the vehicles.
One of the methods of reducing the number of wirings to as small a number as possible and enabling the transmission of many signals is a time division multiplex transmission network.
However, many of the time division multiplex transmission networks which have been put to practical use have a problem in that the transmission time varies greatly depending on the quantity of transmitted data, and hence such systems are not applicable to the control of equipment in the vehicles which are required to send and/or receive control signals in a real time mode.
Namely, the control of various equipment such as the drive control of vehicles has been designed on the condition that the individual control signals are transmitted through respective separate wires. In such a separate wiring system, there is no necessity of considering the time delay required for signal transmission and therefore the delay time for signal transmission has been neglected in the design of the control system.
Therefore, the multiplex communication network, in which the signal transmission time may vary greatly, cannot ensure a satisfactory operation in the train control.
A packet exchange system which is broadly used for telephone exchange or multi-purpose communication is one of the known time multiplex communication networks in which the variation of transmission time is very small.
However, like other conventional time mutliplex transmission networks, the packet exchange system has also a problem in that it generally requires provision or a high technical quality for its transmission stations, as well as peripheral devices, resulting in high cost and wider space for mounting the system.